Wettable injectors for degassing of molten metal

ABSTRACT

Various illustrative embodiments of an apparatus and method for reducing the dissolved hydrogen content of a molten metal alloy are provided. The disclosed embodiments can be utilized for the processing of molten metal alloys such as aluminum, and more particularly, for the removal of dissolved hydrogen from molten metal alloys such as aluminum. Gas permeable diffusers can be employed that are wettable by molten metal. When used as gas injectors, either in combination with ultrasonic oscillation or without, the gas permeable wettable diffusers can provide a high density of ultrafine inert gas bubbles that can be used to rapidly and efficiently reduce the level of dissolved hydrogen within the molten metal.

RELATED APPLICATIONS

This application claims the benefit, and priority benefit, of U.S.Provisional Patent Application Ser. No. 61/394,117, filed Oct. 18, 2010,titled “Wettable Injectors for Degassing of Molten Metal,” thedisclosure of which is incorporated herein in its entirety.

BACKGROUND

1. Field of Invention

This invention relates generally to degassing of molten metal alloys,and more particularly, to an apparatus and method for reducing thedissolved hydrogen content of a molten metal alloy.

2. Description of the Related Art

In industrial applications, molten liquid metal alloys must often bedegassed to remove dissolved hydrogen. In the absence of a degassingtreatment, the monatomic hydrogen that has been absorbed by the moltenalloy, from such sources as atmospheric moisture, will precipitate uponsolidification as pores of diatomic hydrogen gas within the cast metalproduct. Such gas porosity represents a threat to the structuralintegrity of the product because gas porosity cannot be eliminated bysecondary processing such as rolling, forging or extrusion. Because ofthis, the hydrogen content of molten metal alloys is closely monitoredin commercial casting facilities, and means must be employed to reducethe level of dissolved hydrogen within the molten alloy prior to thecasting operation.

SUMMARY

Various illustrative embodiments of an apparatus and method for reducingthe dissolved hydrogen content of a molten metal alloy are providedherein. The disclosed embodiments can be utilized for the processing ofmolten metal alloys such as aluminum, and more particularly, for theremoval of dissolved hydrogen from molten metal alloys such as aluminum.Gas permeable diffusers can be employed that are wettable by moltenmetal. When used as gas injectors, either in combination with ultrasonicoscillation or without, the gas permeable wettable diffusers can providea high density of ultrafine inert gas bubbles that can be used torapidly and efficiently reduce the level of dissolved hydrogen withinthe molten metal.

In an illustrative embodiment, an apparatus for degassing a molten metalalloy is provided. The apparatus can include a container for holding themolten metal and a dispenser capable of dispensing purge gas. A diffusercan be provided that is in fluid communication with the molten metal.The diffuser can be wettable with respect to the molten metal andcapable of receiving purge gas from the dispenser. The dispenser canalso be capable of forming purge gas bubbles and emitting the purge gasbubbles into the molten metal. In another aspect, the diffuser can havea face with a plurality of pores formed thereon. The pores can be influid communication with the molten metal and capable of emitting thepurge gas bubbles into the molten metal. In certain embodiments, theaverage diameter of the pores on the diffuser face is not greater than200 microns. The molten metal can be aluminum. The molten metal can alsocomprise other metal alloys. The molten metal can contain dissolvedhydrogen gas, and the purge gas bubbles can remove the dissolvedhydrogen gas from the molten metal. In another aspect, the apparatus canalso include an ultrasonic oscillator in direct mechanical communicationwith the diffuser. The ultrasonic oscillator can be disposed adjacent tothe diffuser such that the diffuser lies within a sonicated field of theoscillator. The ultrasonic oscillator can also oscillate both below andabove the cavitation power required for the molten metal.

In another illustrative embodiment, a method for degassing a moltenmetal is provided. A molten metal can be provided with hydrogen gasdissolved therein. A purge gas can be introduced into a diffuser. Thediffuser can be wettable with respect to the molten metal. Purge gasbubbles can be formed at the diffuser-molten metal interface. Thedissolved hydrogen gas can be transferred from within the molten metalto the purge gas bubbles, such that the concentration of dissolvedhydrogen gas in the molten metal is reduced.

In another illustrative embodiment, a method for degassing a moltenmetal containing dissolved hydrogen gas is provided. A face of adiffuser can be wetted with the molten metal. A purge gas can be flowedthrough a plurality of pores in the face. In certain embodiments, thepores can have a pore size in the range from approximately 2-200microns. Purge gas bubbles can be produced at the pores in the diffuser.The purge gas bubbles can be emitted from the pores and into the moltenmetal. The dissolved hydrogen in the molten metal can be transferred tothe purge gas bubbles, such that the concentration of the dissolvedhydrogen gas in the molten metal is reduced. In another aspect, anultrasonic oscillator can be provided that is in direct mechanicalcommunication with the diffuser. The ultrasonic oscillator can alsooscillate both below and above the cavitation power required for themolten metal.

It is to be understood that the subject matter herein is not limited tothe exact details of construction, operation, exact materials, orillustrative embodiments shown and described, as modifications andequivalents will be apparent to one skilled in the art.

Accordingly, the subject matter is therefore to be limited only by thescope of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front perspective view of an apparatus for reducing thedissolved hydrogen content of a molten metal alloy according to certainillustrative embodiments set forth herein.

FIG. 2 is a front view of a contact angle (θ) at the liquid-vapor andsolid-liquid interface for a liquid droplet according to certainillustrative embodiments set forth herein.

FIG. 3A is a front perspective view of a purge gas bubble formed from awettable injector according to certain illustrative embodiments setforth herein.

FIG. 3B is a front perspective view of a purge gas bubble formed from anon-wettable injector according to certain illustrative embodiments setforth herein.

FIG. 4 is a top view of a diffuser according to certain illustrativeembodiments set forth herein.

FIG. 5 is a front perspective view of an apparatus for reducing thedissolved hydrogen content of a molten metal alloy having a sonotrodedisposed adjacent the diffuser according to certain illustrativeembodiments set forth herein.

FIG. 6 is a front perspective view of an apparatus for reducing thedissolved hydrogen content of a molten metal alloy having a sonotrodedisposed in direct mechanical communication with the diffuser accordingto certain illustrative embodiments set forth herein.

FIG. 7 is a front perspective view of an apparatus for reducing thedissolved hydrogen content of a molten metal alloy having a sonotrodewith a retaining cap disposed thereon according to certain illustrativeembodiments set forth herein.

DETAILED DESCRIPTION

Various illustrative embodiments of an apparatus and method for reducingthe dissolved hydrogen content of a molten metal alloy are providedherein. In certain illustrative embodiments, the apparatus and methodcan employ gas permeable ceramic diffusers that are wettable by moltenmetal. The gas permeable diffusers can function as wettable injectors toprovide ultrafine inert gas bubbles to the molten metal to reduce thelevel of dissolved hydrogen within the metal. In certain illustrativeembodiments, the diffusers can be used in combination with ultrasonicoscillation to increase the dispersion of the gas bubbles in the metal.The dissolved hydrogen concentration can preferably be reduced to lessthan about 0.4 ml/100 gm at standard temperature and pressure.

In an illustrative embodiment, an apparatus 5 can include a container 10for holding a molten metal alloy 20 and a dispenser 30 capable ofdispensing purge gas into the molten metal alloy 20. The molten metalalloy 20 can comprise, for example, an aluminum alloy or other similarmetal alloy. The purge gas can comprise argon gas or other similar inertgases. In certain embodiments, a small percentage of chlorine gas canalso be included with the purge gas, as needed, to increase theeffectiveness of the purge.

A diffuser 40 can be positioned adjacent to dispenser 30 and utilized toinject and disperse the purge gas into molten metal alloy 20. In certainillustrative embodiments, diffuser 40 can receive the purge gas fromdispenser 30, form a plurality of purge gas bubbles 50, and then emitpurge gas bubbles 50 into molten metal alloy 20. The dissolved hydrogenin molten metal alloy 20 will preferably diffuse through the interfacesof purge gas bubbles 50 as bubbles 50 pass through molten metal alloy20.

In certain illustrative embodiments, diffuser 40 can have a face 60 witha plurality of pores 70 formed thereon, each pore having a lip 75 formedat its interface with face 60. Diffuser 40 can be gas permeable, suchthat purge gas bubbles 50 can be emitted into molten metal alloy 20through pores 70. Diffuser 40 is preferably in at least partial fluidcommunication with molten metal alloy 20, which means that face 60 ofdiffuser 40 can directly contact the fluid of molten metal alloy 20.Pores 70 are also preferably in fluid communication with molten metalalloy 20, which means that to some extent, the surface area near the lip75 of any pore 70 also directly contacts molten metal alloy 20.

Various parts of apparatus 5, including but not limited to dispenser 30and diffuser 40, can be constructed from a material that includes awettable ceramic material such as titanium diboride (TiB₂) or siliconcarbide (SiC) to produce and inject a fine dispersion of purge gasbubbles 50 in molten metal alloy 20. To the extent that purge gasbubbles 50 are emitted through pores 70, the material from which pores70 are formed can also preferably be constructed from a wettablematerial. Purge gas bubbles 50 can provide increased degassing efficacyand can degas molten metal alloy 20 in shorter times than can beaccomplished using conventional rotary nozzle methods, and can alsoremove dissolved hydrogen from greater volumes of molten metal alloy 20than can be treated using non-wettable gas injectors.

Wettable generally means that the material 41 from which the diffuser 40is constructed is capable of a contact angle of less than 90 degrees toa drop of the molten metal alloy 20. As illustrated in FIG. 2, thecontact angle (θ) is the angle at which the liquid-vapor interface(between alloy 20 and purge gas 51) meets the solid-liquid interface(between diffuser material 41 and alloy 20). The tendency of any drop ofthe molten metal 20 to spread out over a flat, solid surface increasesas the contact angle decreases. Thus, the contact angle provides aninverse measure of wettability. A contact angle less than 90° (lowcontact angle) usually indicates that wetting of the surface isfavorable, and the fluid drop will spread over a large area of thesurface. A contact angle greater than 90° (high contact angle) generallymeans that wetting of the surface is unfavorable so the fluid willminimize contact with the surface and form a compact liquid droplet.

In certain illustrative embodiments, the effectiveness of apparatus 5can be increased if apparatus 5 is used in combination with ultrasonicvibration. Diffuser 40 can be placed within the ultrasonic field of asonotrode 90 (See FIG. 5), or alternatively, used as part of sonotrode90 itself. In the latter configuration, an ultrasonic oscillator 80(operating either below or above the cavitation power for molten metal20) can be fitted with diffuser 40 at the end of sonotrode 90 (See FIG.6). In certain illustrative embodiments, ultrasonic oscillator 80 can beused to provide ultrasonic energy to molten metal 20 in the vicinity ofdiffuser 40 and increase the dispersion of purge gas bubbles 50 inmolten metal alloy 20. For example, oscillator 80 can operate aboveand/or below the cavitation power for molten metal alloy 20. Cavitationpower refers to the amount of power needed to create cavities withmolten metal alloy 20. Oscillator 80 can have sonotrode 90 connectedthereto or disposed adjacent thereto. In certain illustrativeembodiments, oscillator 80 and sonotrode 90 can be disposed adjacent tothe diffuser (FIG. 5), or alternatively, oscillator 80 and sonotrode 90can be disposed to directly contact the diffuser (FIG. 6). Oscillator 80and sonotrode 90 can also surround all of, or part of, dispenser 30 toprovide ultrasonic vibration to the diffuser 40 (FIG. 6). Sonotrode 90can be exposed to ultrasonic vibration from oscillator 80, and thenassist in transferring this vibratory energy to molten metal 20.Sonotrode 90 can be constructed from a wettable material such as TiB₂ orSiC or from a refractory metal.

In certain illustrative embodiments, a retaining cap 100 can be utilizedto secure diffuser 40 in a position in direct mechanical communicationwith the sonotrode 90. (See FIG. 7). Retaining cap 100 can have anorifice 110 formed therein that allows purge gas bubbles 50 to exitdispenser 30, pass through diffuser 40 and orifice 110, and enter moltenmetal alloy 20. Retaining cap 100 can be securely affixed to sonotrode90 such that diffuser 40 cannot be misaligned or substantially displaceddue to ultrasonic vibration. Retaining cap 100 can be removable fromsonotrode 90 such that diffuser 40 can be replaced, if desired.

In certain illustrative embodiments, dispenser 30 can extend into theinterior region of sonotrode 90 and deliver purge gas to the diffuser 40(see FIG. 6). In certain illustrative embodiments, diffuser 40 can bemechanically or chemically bonded to sonotrode 90, or alternatively,sonotrode 90 can be fabricated entirely from wettable ceramics such asTiB₂, such that diffuser 40, including face 60 and pores 70, can all besubject to ultrasonic vibration.

Methods for reducing the dissolved hydrogen content of molten metalalloy 20 are also provided herein. In an illustrative embodiment, apurge gas can be introduced into diffuser 40. Diffuser 40 can bewettable with respect to molten metal 20. Purge gas bubbles 50 can beformed with diffuser 40. Purge gas bubbles 50 can be emitted from pores70 of diffuser 40 and into molten metal 20 at a contact angle of lessthan 90° to molten metal 20. The dissolved hydrogen gas in molten metal20 can diffuse into purge gas bubbles 50 such that all or substantiallyall of the dissolved hydrogen gas is removed from molten metal 20.

In an illustrative embodiment, face 60 of diffuser 40 can be wetted withmolten metal 20. Purge gas can be flowed through a plurality of pores 70in face 60. In an illustrative embodiment, pores 70 can have a pore sizein the range from about 2-200 microns. Purge gas bubbles 50 can beproduced at a number of pores 70, with each bubble 50 initiating as ahemi-spherical bubble with a diameter related to the diameter of theparticular pore 70 from which it emerged. Purge gas bubbles 50 can beemitted from diffuser 40 and injected or dispersed into molten metal 20.The dissolved hydrogen gas can diffuse into purge gas bubbles 50 and canreduce the amount of dissolved hydrogen gas in molten metal 20. Also,diffuser 40 can be oscillated below and/or above the cavitation power ofmolten metal 20 to assist in dispersing purge gas bubbles 50.

In certain illustrative embodiments, the diameter of any particular pore70 will affect the diameter of the purge gas bubble 50 emerging fromthat particular pore 70 (see FIGS. 3A and 3B). If diffuser 40 iswettable by molten metal 20 such as, for example, liquid aluminum, theinitial diameter of the hemispherical bubble 50 emerging from pore 70will be set by the pore diameter. This pore diameter can therefore beselected by appropriate sizing of the particle size and volume fractionof the fugitive binders used in the fabrication of diffuser 40. Forexample, smaller pore diameters can result in an increased number ofbubbles 50 each having a smaller size, but an overall increase in totalsurface area of bubbles 50 within molten metal alloy 20. Significantly,as the Stokes' velocity of bubbles 50 rising through a Newtonian fluidis reduced at smaller bubble diameters, the residence time of bubbles 50within molten metal alloy 20 can likewise be increased. The combinationof these two effects, an increased interfacial area between the purgegas and molten metal 20, and a decreased rise speed of bubbles 50 intheir transit through molten metal 20, provide for greater opportunityfor diffusion of hydrogen from molten metal 20 to the purge gas. Anillustrative example of a wettable diffuser 40 is shown in FIG. 4,wherein diffuser 40 has been fabricated from a titanium diboride (TiB₂)material so as to have pores 70 on its face 60 that are gas permeable.TiB₂ can be sintered with fugitive binders (such as graphite) to yieldconnected porosity and also provide sufficient resilience to withstandthe rigors of ultrasonic vibration. Other representative examples ofwettable materials include SiC. Wettable diffuser 40 can provide acontinuous stream of purge gas bubbles 50 into molten metal 20 whendiffuser 40 is in fluid connection with a flow of inert purge gas.

The reduced diameter of bubbles 50 formed from diffuser 40 constructedof a wettable material according to the presently disclosed subjectmatter (see, e.g., FIG. 3A) can preferably increase the surface tovolume ratio of the purge gas and can promote longer residence times forbubbles 50 within molten metal alloy 20. These two effects can enhancethe kinetics of dissolved hydrogen diffusion into the inert gas. If apurge gas injector was constructed of non-wettable materials (such assome ceramics and transition metals), the diameter of the hemisphericalbubble cap that forms would be determined not by the inner diameter ofpore 70 from which bubble 50 emerges, but rather by the outer diameterof the pipe encompassing pore 70 (see, e.g., FIG. 3B). As a consequenceof this phenomenon, the benefit of ever smaller physical pores 70 intoany non-wetting purge gas dispenser is limited, as the effective size ofthe hemispherical gas bubbles 50 formed on the surface of suchnon-wetting purge gas dispenser would likely be more dependent on thediameter of the dispenser itself, rather than the diameter of pores 70from which bubbles 50 emerge.

The embodiments of a degassing apparatus and method provided hereinutilize minimal volumes of inert gas, thus reducing gas cost. Also,enrichment with chlorine gas can be decreased or avoided, which reducesenvironmental concerns and saves on maintenance in the exhaust systems.Also, removal of hydrogen using decreased flows of purge gas can reducedross formation during processing, which directly reduces metal loss andindirectly reduces dross reclamation costs. Also, rapid degassing mayallow for effective in-trough degassing, thus reducing the need fordraining and/or flushing of large rotary head treatment boxes duringalloy changes and the associated losses in productivity.

It is to be understood that the subject matter herein is not limited tothe exact details of construction, operation, exact materials, orillustrative embodiments shown and described, as modifications andequivalents will be apparent to one skilled in the art. Accordingly, thesubject matter is therefore to be limited only by the scope of theappended claims.

1. An apparatus for degassing a molten metal, the apparatus comprising:a container for holding the molten metal; a dispenser capable ofdispensing purge gas; and a diffuser in fluid communication with themolten metal from the container, the diffuser being wettable by themolten metal and capable of receiving the purge gas from the dispenser,forming purge gas bubbles from the purge gas, and emitting the purge gasbubbles into the molten metal.
 2. The apparatus of claim 1, wherein thediffuser has a face with a plurality of pores formed thereon, the poresbeing in fluid communication with the molten metal and capable offorming purge gas bubbles from the purge gas and emitting the purge gasbubbles into the molten metal.
 3. The apparatus of claim 2, wherein theaverage diameter of the pores is not greater than 200 microns.
 4. Theapparatus of claim 1, wherein the molten metal is aluminum.
 5. Theapparatus of claim 1, wherein the molten metal contains dissolvedhydrogen gas and the purge gas bubbles are capable of removing thedissolved hydrogen gas from the molten metal.
 6. The apparatus of claim1, further comprising an ultrasonic oscillator disposed adjacent to thediffuser such that the diffuser lies within a sonicated field of theoscillator.
 7. The apparatus of claim 6, wherein the ultrasonicoscillator is in direct mechanical communication with the diffuser. 8.The apparatus of claim 6, wherein the ultrasonic oscillator is operablebelow or above the cavitation power required for the molten metal. 9.The apparatus of claim 6, wherein the diffuser oscillates below thecavitation power required for the molten metal.
 10. The apparatus ofclaim 6, wherein the diffuser oscillates above the cavitation powerrequired for the molten metal.
 11. The apparatus of claim 1, wherein thecomposition of the diffuser includes a wettable material comprisingtitanium diboride.
 12. The apparatus of claim 1, wherein the compositionof the diffuser includes a wettable material comprising silicon carbide.13. A method of degassing a molten metal, the method comprising:providing a molten metal alloy with hydrogen gas dissolved therein;introducing a purge gas into a diffuser, the diffuser being in fluidcommunication with the molten metal at a diffuser-molten metal interfaceand wettable with respect to the molten metal; forming purge gas bubblesat the diffuser-molten metal interface; injecting the purge gas bubblesfrom the diffuser into the molten metal; transferring the dissolvedhydrogen gas from the molten metal to the purge gas bubbles; andreducing the concentration of dissolved hydrogen gas in the moltenmetal.
 14. A method of degassing a molten metal containing dissolvedhydrogen gas, the method comprising: contacting a face of a diffuserwith the molten metal; wetting the face of the diffuser with the moltenmetal; flowing a purge gas through a plurality of pores in the face, thepores having a pore size in the range from 2-200 microns; producingpurge gas bubbles at the pores; emitting the purge gas bubbles from thepores and into the molten metal; transferring the dissolved hydrogen gasfrom the molten metal to the purge gas bubbles; and reducing theconcentration of the dissolved hydrogen gas in the molten metal.
 15. Themethod of claim 14, further comprising oscillating the diffuser below orabove the cavitation power of the molten metal.
 16. The method of claim14, further comprising disposing the diffuser within the sonicated fieldof an oscillator.
 17. The method of claim 14, further comprisingattaching the diffuser to the sonotrode of an oscillator.